Hi there and thanks for your question.
I'm so sorry to hear that this happened to your little kitten.
It will be hard to say for sure what happened to the little guy but I can give you a few possibilities.
If he had a severe case of fleas it is possible that he had severe anemia. Fleas will take blood meals from animals and kittens do not have a lot of reserves to fight this.
A warm bath is not likely to have caused any problems. However, if you did you an over the counter flea product then many of these are very hard on kittens and can possibly cause death.
It is also possible that the little guy had another illness. He could have been born with feline leukemia or FIV. Or he could have been born with hypertrophic cardiomyopathy which is a heart condition which can cause sudden death.
Thank you for trying to give this kitten a loving home. I am so sorry that he didn't make it.
I hope this information helps! If you have more questions regarding this problem, just hit reply. If I am online I will respond right away. Otherwise I will respond as soon as I come online again. Or, if my answer is sufficient, then please hit accept. Thanks!
Dr. Marie.
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would highly advise contacting your regular veterinarian.
Veterinarian
Compassionate licensed veterinarian with >20 years experience with cats, dogs and pocket pets